1. Field of the Invention
This invention relates to gas turbine construction and more particularly to loosely mounted vibration dampers used with gas turbine blades exposed to high temperatures.
2. Description of the Prior Art
The construction of blades for rotating equipment such as gas turbine or compressor wheels has always required precise engineering to assure that the blades are properly secured to the wheel and that an intolerable amount of vibration does not appear in the blades during operation. Turbine blade design is generally more critical than compressor blade design since turbine blades are exposed to hot gases and experience high stress levels which result from centrifugal, aerodynamic and vibratory loadings on the blades.
Generally speaking, turbine blades tend to change dimension under load in three different directions; axially--along the axis of rotation of the turbine wheel, radially--in the plane of the turbine wheel and tangentially--in the plane of the turbine wheel rotation.
The axial growth presents a relatively small concern because the blades can, for example, be held tightly by side plates attached to the turbine wheel. As the wheel expands and contracts, the side plates and the turbine blades experience corresponding dimension changes and no axial loosening of the blades occurs. Further, there is an axial clearance between adjacent turbine wheels and the change in axial dimension of a given wheel due to thermal growth does not result in any physical interference between adjacent turbine wheels.
Similarly, the radial growth presents a relatively small problem. Turbine blades are often attached to turbine wheels by the familiar fir tree attachment means wherein the contour of the root of the blade conforms closely to the contour of the mating surfaces in the wheel. As the wheel is rotated, centrifugal loading on the blade causes the blade root to maintain a positive contact a bearing surfaces common to the blade and the wheel.
The tangential growth is more difficult to accommodate. Unless an appropriate room temperature clearance is left between the platforms of adjacent turbine blades, the blades will be physically restrained from expanding in the tangential direction when they are heated sufficiently. Jamming or mechanical interlocking of the platforms of adjacent turbine blades and the consequential high stress levels can be eliminated by providing tangential clearance between adjacent blades; however, the blades then tend to vibrate during operation.
Tangential vibrations are sometimes controlled by constructing tip shrouds on the blades. The shrouds connect the tip sections of the adjacent blades and tend to damp and make more consistent the tangential vibrations which occur in the blades. As the gas turbine engine performance has improved, the temperature to which the turbine blades are exposed has increased and the shroud concept has become less attractive because cracks occur in the shroud section of the blade and propagate downward through the blade, resulting in premature failure of the blade. Further, turbine blade shrouding is conceptually undesirable because the diameter of the section of the engine where the shrouding is required tends to increase; also, the shrouding results in a high concentration of mass at a location which is relatively distant from the centerline of rotation, resulting in structural limitations in the engine. Therefore, a trend towards shroudless turbine blades exists. Since the shroudless blade is particularly susceptible to tangential vibration in the airfoil section of the blade, various devices have been resorted to in an attempt to minimize such vibrations. One of the accepted vibration damping schemes is a toggle device. A toggle is a nonstructural member which is held in physical contact with a turbine blade by centrifugal force during rotation of the turbine engine. The toggle reduces the vibratory action of the blade by imposing a retarding force due to friction on the blade at the point of contact between the toggle and the blade. When turbine blades are damped by toggles, each blade tends to vibrate at a frequency which is independent of that in the adjacent blades and the resulting stress levels in some blades becomes high with respect to that in other blades in the same wheel. Also, toggles impose relatively high centrifugal loads on their retaining structure. Further, space limitations impose restrictions on the mass of a toggle which in turn limits the damping force imposed on the blades. Therefore, engine manufacturers are continually searching for alternate inexpensive vibration dampers for use in high temperature, gas turbine applications.
A similar turbine blade damper is shown in U.S. Pat. No. 3,666,376 having an X-shaped construction.